In recent years control systems have assumed an increasingly central role in the advancement of modern technological society. Virtually every aspect of our daily lives is influenced by some type of control system. Control systems are also ubiquitous throughout all sectors of industry. Fields such as quality control of manufacture products, space and military weapons technology, robotics, computer control, automatic assembly lines, transportation systems, automobile engine control, and many others, all employ a theory of automatic controls. As an example, a control system for controlling a robot arm is disclosed in U.S. Pat. No. 4,488,242. Needless to say, this list is growing at an ever increasing rate.
In general, the objective of any closed-loop control system is to regulate the outputs of a "plant" in a prescribed manner by the inputs through the elements of the control system. The "plant" of a control system is defined within the context of this application as that part of the system to be controlled. The inputs to the plant are called the "actuating signals", and the outputs are known as the "controlled variables".
To obtain satisfactory response characteristics in a control system, an additional component--frequently referred to as a "compensator" or a "controller"--is connected within the control loop. While a variety of compensator designs exist, one widely adopted approach utilizes proportional, integral, and derivative compensator elements to develop the actuating signal. Examples of control systems that employ proportional-integral-derivative (PID) compensators are found in U.S. Pat. Nos. 4,679,136 and 4,861,960.
Control systems can be classified in two ways: Continuous-data systems and sampled-data systems. A continuous-data system is one in which the signals at various parts of the system are all continuous functions of time. Sampled-data control systems differ from continuous-data systems in that a signal (or signals) is measured or known only at specific, discrete instants of time.
In an analog sampled-data control system a sample and hold circuit is typically used to sample a system parameter at discrete points in time. The circuit then holds this value for one sample period. Thus, the output of the sample and hold represents or approximates the continuous time varying value of the sampled system parameter. If a digital computer is used in the control of a sampled-data control system (as is commonly the case), then such systems are called digital sampled-data control systems. In a typical digital sampled-data control system, an analog-to-digital (A/D) converter is used to sample a continuous signal. The digital computer then calculates a control value which is subsequently converted to an actuating signal by a digital-to-analog (D/A) converter. For such a system the D/A converter performs the hold function.
Because digital computers provide many advantages in terms of size, flexibility and cost, computer control has become increasingly popular in recent years. The problem with digital control systems, however, is that they tend to have less desirable frequency response characteristics when compared to their analog counterparts. In particular, digital compensators suffer from an exceedingly large phase loss as a function of operating frequency. The extreme phase loss associated with digital control systems compels the circuit designer to either lower the control loop bandwidth or compromise system stability. Hence, the overall system performance is undermined.
As a consequence, there remains an unsatisfied need to reduce the phase loss associated with digital control systems.